1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an apparatus and method for cleaning a cathode assembly of an ion source chamber used in ion implantation equipment.
2. Discussion of Related Art
Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. FIG. 1 is a block diagram of an ion implanter 100 including an ion source chamber 102. A power supply 101 supplies the required energy to source 102 which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes 104 and formed into a beam 95 which passes through a mass analyzer magnet 106. The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer. Ions of the desired species pass through deceleration stage 108 to corrector magnet 110. Corrector magnet 110 is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to provide a ribbon beam targeted toward a work piece or substrate positioned on support (e.g. platen) 114. In some embodiments, a second deceleration stage 112 may be disposed between corrector magnet 110 and support 114. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy.
The ion source chamber 102 typically includes a heated filament which ionizes a feed gas introduced into the chamber to form charged ions and electrons (plasma). The heating element may be, for example, a Bemas source, an indirectly heated cathode (IHC) assembly or other thermal electron source. Different feed gases are supplied to the ion source chamber to obtain ion beams having particular dopant characteristics. For example, the introduction of H2, BF3 and AsH3 at relatively high chamber temperatures are broken down into mono-atoms having high implant energies. High implant energies are usually associated with values greater than 20 keV. For low-energy ion implantation, heavier charged molecules such as decaborane, carborane, etc., are introduced into the source chamber at a lower chamber temperature which preserves the molecular structure of the ionized molecules having lower implant energies. Low implant energies typically have values below 20 keV. When a particular feed gas is supplied to source chamber 102 to produce a desired ion species, additional unwanted species, either ions or neutrals, may also be produced. These unwanted species typically have low vapor pressure and may condense and adhere to the interior surfaces of the source chamber. For example, when phosphine (PH3) is fed into the source chamber, phosphorous (P) deposits may form on the chamber walls. When heavy molecules such as decaborane and carborane are fed into the source chamber, unwanted deposits on the source chamber walls and electrodes is more prevalent. These solid deposits may change the electrical characteristics (voltage instability) of the chamber walls and possibly interfere with the ion source aperture from which the ions are extracted, thereby causing unstable source operation and non-uniform beam extraction.
One method used to clean the ion source chamber includes the introduction of a cleaning gas such as, for example nitrogen triflouride (NF3) or sulfur hexaflouride (SF6) which etches away the unwanted deposited material via plasma-enhanced chemical reaction. These gases are supplied to the ion source chamber at high flow rates thus maintaining high-pressure to effectively clean the interior of the chamber. However, these high flow rates cause the fluorine containing gases to diffuse and travel to other components near the ion source chamber. In particular, these gases may diffuse to the cathode assembly of the IHC. Because the cathode assembly is active during cleaning operation to increase the temperature within the source chamber and the reaction of the cleaning gases, an electro-thermal reaction occurs at the cathode assembly which produces additional deposits being formed on the filament. These filament growths may cause electrical shorts in the IHC and therefore cause equipment downtime for source PM. In addition, because the cleaning gases are introduced into the source chamber at relatively high flow rates, a need exists to reduce these rates in the area of the IHC to reduce unwanted filament growth while not compromising the pressure and flow rates needed for effective cleaning within the ion source chamber. Thus, there is a need for enhancing ion source cleaning to increase efficiency and unnecessary equipment downtime of ion implanters while not negatively impacting source components during the cleaning process. In addition, there is a need to provide a localized inert gas directed toward the filament/cathode region at a relatively low flow rate to extend the source lifetime while not effecting the ion source performance during normal implanter operation.